The present invention relates to an electric power steering apparatus for applying a steering assisting force to a steering mechanism of a vehicle by a brushless motor.
In a related art, there is used an electric power steering apparatus applying a steering assisting force to a steering mechanism by driving an electric motor in accordance with a steering torque applied to a handle (steering wheel) by a driver. The electric power steering apparatus is provided with a torque sensor for detecting the steering torque applied to the steering wheel which is an operating unit for steering, and a target value of current (hereinafter, referred to as “target current value”) to be supplied to the electric motor is set based on the steering torque detected by the torque sensor. Further, an instruction value to be provided to a driving unit of the electric motors is generated by proportional integrating operation based on a deviation between the target current value and a value of current actually flowing in the electric motor. The driving unit of the electric motor is provided with a PWM signal generating circuit for generating a pulse width modulating signal (hereinafter, referred to as “PWM signal”) having a duty ratio in accordance with the instruction value, and a motor driving circuit (hereinafter, simply referred to as “driving circuit”) constituted by using a power transistor made to be ON/OFF in accordance with the duty ratio of the PWM signal, and applies voltage in accordance with the duty ratio to the electric motor. Current flowing in the electric motor by applying the voltage is detected by a current detecting circuit, and a difference between the target current value and the detected current value is used as the deviation for generating the above-described instruction value.
Although as an electric motor (drive source) used in such an electric power steering apparatus, in a related art, a motor with a brush has been frequently used, in recent years, there is also used a brushless motor from a standpoint of promotion of reliability and durability, a reduction in enertia and the like. According to an electric power steering apparatus using a brushless motor, there is normally provided an opening/closing unit (typically, relay) for electrically cutting to separate a driving circuit and a motor as necessary when the driving circuit is failed. In this case, it is preferable that a number of pieces of the opening/closing unit of relays or the like is as small as possible because of a restriction in cost or space and therefore, there is used opening/closing unit of a number of pieces which is necessary minimum for cutting current supplied from the driving circuit to the motor. For example, in the case of an electric power steering apparatus using a brushless motor of 3 phases, there are used 2 pieces of relays as opening/closing unit for cutting supply of currents of 2 phases in 3 phases of currents supplied from the driving circuit to the motor (see JP-A-2001-106098, for example)
According to a constitution minimizing a number of pieces of relays used as opening/closing unit as described above, in current paths of respective phases between a brushless motor and a driving circuit, there are present a phase in which a relay is interposed and a phase in which a rely is not interposed. Therefore, resistance components in a motor-driving circuit system comprising the brushless motor and the driving circuit differ among phases, and according to an electric power steering apparatus having such a circuit constitution, gains and phases of the motor-driving circuit system as transmitting elements constituting an input by a phase voltage (instruction value) to be applied to the brushless motor and constituting an output by a phase current actually flowing in the motor differ among phases. As a result, even when an equal voltage is applied to respective phases of the brushless motor in control, amplitudes and phases of currents flowing in the brushless motor differ among phases. Such a difference among phases of motor currents constitutes a factor of generating a torque ripple in the brushless motor and therefore, a driver feels a strange feeling in steering operation.